In recent years there has been progress in the development of cognitive radio as represented by IEEE 802.22. Cognitive radio performs communication in which radio wave condition in the area of a base station or a mobile station is confirmed, and in accordance with the conditions thereof, parameters such as frequency used in wireless communication and the like are switched. With regard to a frequency band used in the cognitive radio, a frequency band the same as a frequency band used in digital television is assumed. Furthermore, since frequency bands used in digital television differ according to region, technology known as spectrum sensing is essential in order to find a frequency (empty channel) that is not being used in the area.
Spectrum sensing can be classified by two methods: a method using a characteristic of a received signal and a method not using the same. The method that uses the characteristic of a received signal is called fine sensing, and since the characteristic of the received signal is used, the sensitivity is high and the method is suitable for distinguishing micro signals. On the other hand, the method has a characteristic in that speed of detection is slow and it is not possible to detect an unknown signal.
Meanwhile, the method that does not use a characteristic of a received signal is called blind sensing, and since a characteristic of the received signal is not used, detection of signals is fast and it is possible to detect an unknown signal. However, this method has a characteristic of having low detection sensitivity.
Patent Literature 1 discloses that for signal detection, FFT (Fast Fourier Transform) computation is used normally in a digital area, but a Fourier Transform circuit can be implemented also in an analog area.    Patent Literature 1: Japanese Patent No. 3727406